Gas discharge ultraviolet lasers used as light sources for integrated circuit lithography typically are line narrowed. A preferred prior art line narrowing technique is to use a grating based line-narrowing unit along with an output coupler to form the laser resonator cavity. The gain medium within this cavity is produced by electrical discharges into a circulating laser gas such as krypton, fluorine and neon (for a KrF laser); argon, fluorine and neon (for an ArF laser); or fluorine and helium and/or neon (for an F2 laser).
Such a prior art system 1 is shown in FIG. 1, which is extracted from Japan Patent No. 2,696,285. The system 1 shown includes output coupler (or front mirror) 4, laser chamber 3, chamber windows 11, and a grating based line-narrowing unit 7. The line-narrowing unit 7 is typically provided on a lithography laser system 1 as an easily replaceable unit and is sometimes called a “line narrowing package” or “LNP” for short. This prior art unit includes two beam-expanding prisms 27 and 29 and a grating 16 disposed in a Littrow configuration. Gratings used in these systems 1 are extremely sensitive optical devices. A typical grating 16 surface may have 10,000 grooves per inch created in an aluminum layer or layers on a thick glass substrate. These gratings 16 and techniques for fabricating them are described in U.S. Pat. No. 5,999,318, which is incorporated herein by reference. A prior art technique for avoiding distortion of the grating 16 surface is to mount the grating 16 on a metal grating mount made of a material having a small co-efficient of thermal expansion closely matched to the thermal expansion co-efficient of the grating 16 glass substrate. The gratings 16 can deteriorate rapidly under ultraviolet radiation in the presence of oxygen in standard air. For this reason, the optical components of line narrowing units 7 for lithography lasers 1 are typically purged continuously during operation with nitrogen. For example as shown in the above referenced Japanese patent from which FIG. 1 is taken, a nitrogen purge system 40 may include, e.g., a nitrogen bottle 44 containing nitrogen under pressure may, when valve 48 is opened provide nitrogen through a nitrogen purge line 45 under flow control from, e.g., a flow control mechanism 46 into the interior of the enclosure surrounding the line narrowing package 7. As shown in this Japanese patent, the nitrogen gas 45 is injected behind the grating 16 and purposely not flowed across the reflecting facets of the grating 16.
FIG. 2 shows a prior art line-narrowing unit 50 fabricated by applicants' employer, Cymer, Inc., as a part of a prior art line narrowed lithography KrF laser system incorporating such a device. The unit 50 includes, e.g., three beam expanding prisms 58, 60 and 62, a tuning mirror 64 and a grating 16. Note that the nitrogen purge from bottle 44 enters the unit on the back side of the tuning mirror 46 to avoid purge flow directly on the grating 16 face, as shown in the above referenced Japanese patent. In this system 20 the wavelength of the laser beam 6 can be controlled, e.g., in a feedback arrangement in which the wavelength of the beam 6 is measured by a beam wavelength monitor 22 and a computer controller 24 uses the wavelength information to adjust the angular position of the tuning mirror 64 to control the wavelength to a desired value. A wavefront (and thus also bandwidth) control device 70 can be used, e.g., to mechanically bend the grating 16 to make it slightly concave, for example, or slightly convex. This device 70 is described in detail in U.S. Pat. No. 5,095,492 assigned to Cymer. Use of this device 70 can permit, e.g., reduction of the bandwidth somewhat, but it can still go out of specification when the laser is run at high duty cycle.
For many years, designers for line narrowed lasers have believed that distortions of the laser beam could be caused by gas flow near the face of the grating 16. Therefore, laser designers in the past have made special efforts to keep the purge nitrogen from flowing directly on the face of the grating 16. Several examples of these efforts are described in the Japan Patent 2,696,285 referred to above. In the example shown in extracted FIG. 1, the purge flow is directed from N2 gas bottle 44 toward the backside of grating 16 through purge line 45.
Line narrowed ultraviolet laser light sources currently in use in the integrated circuit industry typically produce about 10 mJ per pulse at repetition rates of about 4000 Hz and duty factors (duty cycles) of about 20 percent. Increased integrated circuit production can be achieved at higher repetition rates and greater duty cycles. Applicants' employer is currently selling a 4000 Hz gas discharge lithography laser, including some, e.g., in a MOPA configuration. Higher repetition rates along with higher duty cycle demands and requirements of ever narrower bandwidths, can, even in MOPA configurations, place sever constraints on optical components, e.g., the LNP 50 to be able to maintain consistent narrow bandwidths. A significant portion of the problem arises from the requirements to maintain rather strict parameter controls on the laser beams traveling through the system, e.g., including wavefront control, which can impact the performance of the LNP among other optical and lasing components of the system 2.
A need exists for more reliable line narrowing packages 50 and other optical elements of the laser system 2 to accommodate such requirements as high repetition rate, high duty cycle, bandwidth stability, included not exceeding some specific bandwidth and/or not allowing the bandwidth to go below some lower limit in addition to not exceeding some upper limit, exposure dose stability, and the like for gas discharge lasers of the type disclosed in the present application.
It has also been known to mount the mirror 64 on a mirror mount as shown in FIGS. 6 and 6A using a plurality of, e.g., three, mounting balls, e.g., made of aluminum, including, e.g., two right (as shown in the figure) mounting balls 156 and one left (as shown in the figure) mounting ball 158. The mounting balls 156, 158 may, e.g., be contained in cylindrical pockets, e.g., as shown more clearly in FIG. 9 at 156a and 156b showing a version of an aspect of an embodiment of the present invention, which are formed in the mirror mount 152. The mounting balls 156, 158 may cooperate with spring loaded mounting clips 168 contained respectively on a right (as shown in the figure) elevation wall 162 of the mirror mount 152 and on a left (as shown in the figure) mounting wall 164 of the mirror mount to hold the mirror 154 in place firmly, but without distorting the face of the mirror or its flatness, as will be understood by those skilled in the art. Problems have arisen, however, with such mountings, e.g., induced by thermal stressing of the mirror mount 152 and the mirror 154, which, due to the fact that the former of which is usually made of metal, e.g., aluminum, invar or stainless steel, and the latter of glass, e.g., fused silica, having very different coefficients of thermal expansion will expand differently when heated.
It has been found that the prior art mounting just described can cause several problems most notably in negatively impacting the LNP 50 in performing its functions within the very stringent requirements for, e.g., wavelength and bandwidth accuracy and stability. What has been discovered to be occurring is that differing thermal expansion of the mirror 154 and the mount 154 tends to force the mirror to slide across the mounting ball surfaces, as was the original design intent. However, the glass surface of the mirror 154 has been found to intermittently and unpredictably sometimes adhere to the mounting ball surfaces or at least one of them causing intermittent and unpredictable distortions in the face of the mirror 154 with concomitant disastrous results in the maintenance of wavelength and/or bandwidth stability. An aspect of an embodiment of the present invention is for the purpose of resolving this problem and related species of problems that similarly impact the contribution of the mirror 64 in the LNP in the performance of its functions.
It has also been noted that as optical elements, e.g., the grating 16 is the LNP 50 are being subjected to higher fluence environments grating 16 failures have increasingly been observed. The observation is also that these failures, by in large, are a result of oxidation of the aluminum reflective layer that forms the reflecting surface of the grating. An aspect of an embodiment of the present invention is directed to a solution to this increasingly more significant problem.
A prior art line narrowed gas discharge laser, e.g., a KrF excimer laser or an ArF excimer laser, operating at relatively low average power, typically less than 5 W, will produce a laser beam centered, respectively at about 248 nm or 193 nm with a bandwidth of less than 0.6 pm. Such lasers can run without problems at repetition rates, up to 2000 Hz and even above that as long as average power is below 5 W. More modern lasers with, e.g., a MOPA configuration can operate at repetition rates exceeding 4000 and with even higher power. However, this, combined with duty cycle requirements, and in some cases customers have been known to operate such laser lithography light sources at near 100% duty cycle over long stretches of time, days or weeks. A typical lithography gas discharge laser has a pulse energy of 10 mJ. By way of example, such lasers can be run at 2 kHz in bursts of 200 pulses with pause between bursts of about, e.g., 0.4-0.45 seconds. Such an operation will produce an average power of:
                              P          ave                =                                            10              ⁢                                                          ⁢                              mJ                ·                200                            ⁢                                                          ⁢              pulses                                      0.5              ⁢                                                          ⁢              sec                                =                      4            ⁢                                                  ⁢            W                                              (        1        )            
When the interburst delay is decreased, e.g., with the laser running the same 200 pulse bursts with a 0.1 second interburst delay will have an average power of:
                              P          ave                =                                            10              ⁢                                                          ⁢                              mJ                ·                200                            ⁢                                                          ⁢              pulses                                      0.2              ⁢                                                          ⁢              sec                                =                      10            ⁢                                                  ⁢            W                                              (        2        )            At maximum, the laser is run in continuous mode which, at 2000 Hz and 10 mJ pulse energy, is equivalent to 20 W average power. These factors simply go up with increases in repetition rates, e.g., to and above 4000, increasing such things as the thermal loading in optical components, including particularly the line narrowing package. The increases on thermal loading only serve to make the problems of maintaining precision operation of such elements as the reflecting mirror 64 and the grating 16 more difficult at the very time when the requirements for precision in operations are becoming even more stringent.
FIG. 2A-1 is a side view showing a prior art method of mounting a grating 16 to the floor of the LNP 50 enclosure. In this case, the thick glass substrate of the grating is attached to mounting plate 16A at each of three spots with very short epoxy dots 17A-C. FIG. 2A-2 shows the approximate dot positions at 17A, 17B and 17C. The mounting plate 16A is screwed securely to the floor of the LNP enclosure with two screws 16B and 16C. In this prior art design, the mounting plate 16A is made of invar, which has a co-efficient of thermal expansion close to zero and to that of the grating 16 glass substrate which may be, e.g., ultra low expansion glass and also close to zero in coefficient of thermal expansion. However, the LNP 50 enclosure is aluminum, which has a co-efficient of thermal expansion substantially different from both invar and the grating 16 glass substrate. As a consequence, temperature excursions in the LNP 50 produces bending stresses on the mounting plate 16A which is screwed tightly to the bottom of the LNP 50 enclosure and the plate 16A in turn produces bending stresses on the grating 16 through the short epoxy dots. As long as the temperature excursions are small the dots are flexible enough to prevent significant distortion; however, at high repetition rates in the range of 2000 Hz to 4000 Hz and above and high duty factors, thermal distortion in the grating 16 has been so great as to substantially adversely affect the quality of the laser beam both in terms of bandwidth and wavelength centerline stability, and the problem is only getting worse.
As noted in the above referenced U.S. Pat. No. 6,496,528, the cause of beam quality deterioration with increasing beam energy, the cause was not initially readily apparent, however, applicants recognized that differential thermal expansion was causing an undesirable bending of the grating 16 which was causing, e.g., the deterioration in beam quality. As a result, applicants designed several modifications in the LNP to solve this problem as discussed in the above referenced U.S. Pat. No. 6,496,528.
One such improvement known in the art is shown in FIGS. 3A, B and C, which are top, side and bottom views respectively of a grating mount 16A″ with a single H-Flex joint 121. This mount 16a″ is comprised of invar which, as stated above has a very small thermal coefficient of expansion about the same as the, e.g., Ultra-Low-Expansion (“ULE”) made by Corning, or e.g., fused silia grating 16 glass substrate. The mount 16A″ is about ½ inch thick and has a length approximately equal to the thick glass substrate of the grating 16 to be mounted on it. Holes 120 are cut into the mount 16A″ primarily to reduce its weight. An H-Flex joint 121 is machined into the mount 16A″ as shown in FIG. 4A. Two dog-bone shaped holes 124 are cut into the mount 16A″ producing an “H” shape flex joint 121 so as to leave four flex legs 126 each about 0.060 inch thick. The grating 16 is preferably mounted to the mount 16A″ with three short epoxy dots at locations 128 about 4 mils high and 1 cm in diameter as shown in FIG. 4A.
The mount 16A″ is screwed securely to the bottom of the LNP 50 enclosure using threaded holes 130 shown in FIG. 4C. The H-Flex joint 121 permits thermal expansion and contraction of the LNP 50 enclosure bottom without transmittal of any significant stress to the grating 16. The joint 121 provides very little resistance to small forces in the long direction of the grating 16. The expansion co-efficient is about 0.001 inch per pound, in this long direction, but is extremely strong and resistant to forces in any other direction.
A third embodiment according to the prior art can be described by reference to FIGS. 5A, B, C, D, E and F. FIG. 5A is a top view, FIG. 5B is a side view and FIG. 5C is a bottom view. This mount 16A′″ is similar to the one described above and shown in FIG. 4A. However, this mount 16A′″ has two H-Flex joints 134 and 133 as shown enlarged in FIGS. 5D and 5E respectively. The mount 16A′″ is made of aluminum as is the LNP 50 enclosure. The mount 16A′″ is mounted to the enclosure as described in the previous embodiment. The grating 16 is attached to the mount 16A′″ with three short epoxy pillars as described above at the positions 132A, 132B and 132C as shown in FIGS. 5A, 5D and 5E. The surface of the mount 16A′″ at the epoxy positions (in this as well as the other embodiments) is preferably abraded with #40 grit dry blast alumina to produce a good epoxy surface. The legs of H-Flex joint 134 and H-Flex joint 133 are about 0.030 inch wide. H-Flex joint 133 permits expansion of the mount 16A′″ relative to the grating 16 in the long direction of the grating and H-Flex joint 134 permits expansion in a short direction of the grating 16 as indicated by the drawings. The second H-Flex joint 134 permits the use of aluminum as the mount 16A′″ material, which is less expensive and easier to machine than invar. Preferably, a four-mill shim (not shown) is used in this and the other embodiments when attaching the grating 16 to the mount to assure that the epoxy dots are the correct thickness (i.e., height).
Preferred designs of grating mounts 16A-16A′″ of the prior art take into consideration the material used for the mount. For example, in the FIGS. 3A-D and in FIGS. 4A-C examples, invar is used for the mounts 16A′ and 16A″ which has a thermal co-efficient of thermal expansion similar to the thick ULE glass grating 16 substrate. Therefore, the grating 16 is attached with the three short epoxy dots at two far apart locations to the long part 100 of the mount 16A′ with no attachment to the short part 102 of the mount 16A′. In this example, one end of the long portion 100 of the mount 16A′ is attached to the LNP 50 enclosure and the short portion 102 of the mount 16A′ at the other end of the mount 16A′ is separately attached to the LNP 50 enclosure. The H-flexure joints in the second example 16A″ permits the mount 16 and the LNP 50 enclosure (having significantly different coefficients of thermal expansion) to expand and contract at different rates.
In the FIG. 5A-E examples, the mount 16A′″ is securely attached at two locations to the enclosure housing. This produces no significant stresses since both the mount 16A′″ and the enclosure housing are aluminum. The grating 16 substrate which has a coefficient of expansion substantially different from aluminum is attached to each of the three separate parts of the mount 16A′″ which by reason of the two H-flexure joints 133, 134 are free to move relative to each other. Thus, if in the FIGS. 3A-D and FIGS. 4A-C examples the mount were made of aluminum then in each case the long part of the mount only should be attached to the chamber floor and the grating should be attached to both parts. Also if in FIGS. 5A-E example, invar were used for the mount, the grating should be attached to the long solid part and the mount should be attached to the chamber at points 132A, 132B and 132C.
Turning now to FIG. 6 there is shown a prior art mounting for a maximum reflecting mirror according to the prior art as discussed above. The maximum reflecting mirror, referred to as RMax 150 corresponds, e.g., to the mirror 64 in FIG. 2. In the past, as noted above, this mirror 150 comprised of an almost totally reflecting, i.e., RMax, multiple layer mirror, has been mounted onto a mirror mount 152. In efforts to meet the problems created in the prior art design, applicants tried a variety of alternative materials and combinations of materials for the mounting balls 156, 158, with varying degrees of partial but incomplete success.
Applicants have proposed in the past to carry out active control of the wavefront of a laser produced from a gas discharge, e.g., excimer or molecular fluorine, laser using, e.g., a bending screw and a stepper motor. In the past, this solution may have been too expensive for the amount of benefit to the laser performance being within required specifications. However, as noted above, the requirements are becoming exceptionally more demanding and wavefront, particularly in its impact on line narrowing, e.g., in an eschelle grating type line narrowing unit as described above, is becoming more critical to control. Currently in many lasers in the field, a grating bending screw is set at a compromise position empirically determined between a hot and cold position of the grating and then left alone, so-called set and forget. However, this solution if becoming increasingly less satisfactory and the requirements being placed on the laser systems, particularly, e.g., due to lithography light source requirements, a stepper motor active control may not even be satisfactory. An aspect of an embodiment of the present invention seeks to resolve these issues.
Applicants have also discovered that even with a flexure mounting, the forces exerted on the optical element in simply moving the flexure element can be sufficient to introduce discernable and unacceptable effects on such things as beam profile, wavelength, bandwidth or the like. Therefore another aspect of an embodiment of the present invention is meant to correct this problem.
It is also known to protect laser optics, particularly containing metal halide crystal glass, e.g., MgF2, using a silicon oxyfluoride coating, e.g., fluorine doped fused, i.e., amorphous, silica, as discussed e.g., in U.S. Pat. No. 6,466,365, entitled FILM COATED OPTICAL LITHOGRAPHY ELEMENTS AND METHOD OF MAKING, issued to Maier et al. on Oct. 15, 2002, for purposes of, e.g., providing a protective coating that is adequately transmissive in the VUV range of wavelengths. Such coatings serve, e.g., to prevent the introduction of impurities into the crystal structure voids from the surrounding ambient atmosphere, with resultant damaging effects to the optical performance of the optical element. Similarly, applicant assignee has used such coatings to prevent DUV damage to transmissive optical elements, as discussed, e.g., in United States Published Patent Application No. 2003/0219056 A1, entitled HIGH POWER DEEP ULTRAVIOLET LASER WITH LONG LIFE OPTICS, with inventors Yager et al., published on Nov. 27, 2003, based on an application Ser. No. 10/384,967, filed on Mar. 8, 2003. The coating serves to provide fluorine atoms to replace fluorine atoms removed from, or perhaps displaced in the crystal structure of the fluoride glass due to high fluence of DUV radiation through the glass, e.g., in laser chamber window, thereby preventing fluence related optical damage to the glass of the optical element. Applicants herein propose yet another utilization for such protective coatings in relation to laser optical systems.
It has been known for some time that line narrowing module gratings are subject to failures, which have been identified as relating to oxidation of the metallic reflective coating, e.g., aluminum on the grating surface. Currently coatings on metallic reflecting surfaces exposed to DUV light, e.g., at 193 nm consist of fluoride coating materials, e.g., MgF2, due to the resistance of such materials to damage in the DUV range of fluence and high transmissivity, i.e., transparency, of such coatings. However, such coatings have not been successful in protecting gratings exposed to high fluence of DUV light. An aspect of an embodiment of the present invention addresses this problem.
It is well known that organic materials, e.g., certain polymers, cannot be used in portions of laser systems according to those employing various embodiments of the present invention. Not only do they typically not react well to UV light, but also are highly chemically reactive to laser gases, e.g., fluorine or chlorine. However, in certain applications in such laser systems in the optical path, but sealed from exposure to the reactive gases, organic materials, e.g., Teflon, may be necessary for use due to the large differences in price between such seals and the, e.g., metal c-ring seals used in contact with the reactive laser gases. An aspect of an embodiment of the present invention deals with the solution of certain problems arising from using such organic materials in seals in the optical path, e.g., external to the laser chamber.